I have a MacBook Pro late 2008 (2.4GHz Core 2 Duo with 4GB memory) and am considering getting a second 24" monitor. I understand having a dual monitor setup may decrease performance, but how much would that be? How can I know if a 24" second monitor would in fact have significant impact on my MBP performance without having to go to a store and test it myself?
https://support.apple.com/kb/HT3246 talks about memory usage for various Mac products, including the late-2008 MacBook Pro you are using. Your MacBook allocates a portion of its RAM for video. You can hit the wall with that if you're rendering lots of video, but as far as I can tell, your GPU is discrete (ie. not part of your CPU, so multiple monitors won't affect your CPU performance) and the amount of memory allocated to video doesn't change upon hooking up a second monitor (so you won't decrease the quantity of RAM available to your applications).
I have the same setup - a 24" iiyama Prolite and a Early-2010 Macbook Pro with 2.4ghz dual core and 8gb of RAM. Originally I had 4, but I upped to 8.
I've noticed almost no change in performance since adding the screen - even running heavy apps like Photoshop, there is very little to no lag - and for general web usage (Chrome, Spotify, Forklift and Textmate) there is absolutely no change and no lag.
I'm starting to think in hindsight that it was unnecessary to upgrade the RAM (although this was a £30 job from Crucial) - the only time where I notice the addition is in boot times.
I have the same model and have had it HDMI ported to my 60in Panasonic for several months acting as my main media center. I've also used it playing Battlefield Bad Company 2 amongst other games and though I have experienced slight lag, it was more than likely due to only have 256MB vRam in that model.
Short answer: No change in performance noticed by being dual monitored while watching movies/tv shows/working in MS Office.
*(I know it's offtopic but..) I have to disagree with the above poster, I have noticed significant differences in my MB 5,1 after upgrading to 8GB ram, particularly with running VMWare/Parallels applications. Where it sits now, I am capable of running OSX 10.6+ with various apps like iTunes and Safari, Visual Studio 2010 in a windows environment, and Ubuntu 11.04 with several apps running such as Xchat and Steam all at the same time without significant lag.